1. Field of the Invention
The invention relates to a spray device for the electrostatic coating of articles with coating material, particularly powdered coating material.
2. Description of the Prior Art
In principle, there are two different methods of electrostatic coating. One is the charging method, in which a high voltage is conducted from a central source of high voltage to one or more spray devices. In the other method, the high voltage is produced in the spray device by means for providing a voltage. The present invention relates to this second method. In principle, all spray devices based on this second method which are available on the market today operate as follows:
For manual operation the spray device has the shape of a pistol, while for automatic coating systems it frequently has the shape of a short bar. The spray device is fed a high frequency alternating current of low voltage. This low voltage is transformed at the inlet of the spray device to several thousand volts by a transformer. This medium-high voltage is then multiplied in the spray device in a voltage multiplying and rectifying circuit to the final voltage required on the electrodes used for electrostatic charging of the coating material and is converted into direct current. Such a circuit customarily consists of a voltage-multiplier cascade of diodes and capacitors, also known as a Greinacher circuit from the name of its inventor. The advantage of this system is that the high final voltage is reached only in the spray device and specifically the highest voltage value is reached only at the end of the spray device where the electrodes are located.
A device of this type which is marketed by applicant's assignee has an elongated body through which a channel for the coating material extends in longitudinal direction to a delivery opening which emerges from the body. In the body, parallel to and outside of the channel, is an electric transformer which has a bar-shaped core and at least one primary winding and one secondary winding surrounding the core. At least one electrode is mounted in the delivery opening of the body for the electrostatic charging of the coating material conducted through the channel. An electric voltage-multiplying and rectifying circuit is arranged within the body and connects the transformer to the electrodes. An electric connecting device connects the primary winding of the transformer to a source of current.
The generation of high voltage by the method described above has limitations. In coating narrow tubes, nozzles and other hollow bodies whose inside diameter is not substantially greater than the outside diameter of the spray device, the electric and magnetic fields in the spray device are greatly impeded by the grounded articles to be coated. In particular, bar transformers, which are preferred because of considerations of space, change their output considerably under these influences. This is due to the fact that the open magnetic flux of the magnetic field is disturbed by the article to be coated.
Attempts have been made to circumvent these disadvantages by means of transformers of various design which have a completely closed magnetic flux circuit. These efforts have, however, up to now failed in practice due to the size of these units and other technical considerations.
U.S. Pat. No. 3,608,823, which corresponds essentially to West German Pat. No. 20 65 699, shows a pistol-shaped spray device at whose upstream rear end an annular transformer concentrically surrounds the channel for the coating material. This patent also describes voltage multiplying and rectifying circuits in the form, on the one hand, of a voltage-multiplier cascade, and on the other hand, of a voltage divider. The parts of this circuit are distributed uniformly around the channel.
West German Unexamined Application for Patent No. OS 30 08 843 shows a pistol-shaped spray device in which an oscillator that is fed with a low DC voltage, a transformer and a cascade circuit are each formed as a hollow cylindrical annular unit and concentrically surround the channel for the coating material. Concentrically surrounding structural groups can, however, produce disadvantageous electric and magnetic field effects, must be specially manufactured, and can in each case be used only for a spray device of a given size, while the use of commercial electrical components would be considerably cheaper.
U.S. Pat. No. 3,731,145 shows a pistol-shaped spray device having a unit consisting of a transformer and a DC-voltage/AC-current converter arranged in its upstream end and having a voltage multiplier unit arranged over the channel for the coating material. The transformer has a cup core.
U.S. Pat. No. 4,196,465 shows a pistol-shaped spray device which consists essentially of two parts which lie with parallel axes over one another and can be detached from each other. The channel for the coating material passes through the upper part. Within the lower part are arranged the electrical components for producing the voltage for the electrodes which serve for the electrostatic charging of the coating material. These components consist essentially of an oscillator for converting the DC voltage of a source of voltage into an AC voltage, followed by a transformer for transforming the oscillator voltage to a medium-high voltage. A multi-step cascade circuit is connected to the secondary winding of the transformer for further increasing the voltage and converting it to a given high DC voltage value which is fed to the electrodes.